In certain areas of the world, such as many countries of Europe, mobile telephones have become very popular. Large numbers of mobile telephones are carried by people in all walks of life as they go about their daily business. For the vast majority of the time, these telephones are either turned off completely, or are in standby mode, that is, they are powered and registered with a base station but are not actively communicating.
Many applications exist in which location-dependent data needs to be collected. Examples of such applications are weather monitoring, meter reading, evaluation of traffic conditions. Each of these applications will be described in greater detail below, but each requires a large amount of data to be collected from geographically dispersed sites. It would be expensive to install and maintain a dedicated communications system for collecting the required data from each of these various sources.